1. Technical Field
The present disclosure relates to a solid-state imaging device.
2. Description of the Related Art
With the growing popularity of digital cameras, camera-equipped mobile phones, vehicle cameras, etc., there is demand for miniaturization, higher pixel resolution, and sensitization of solid-state imaging devices. In order to sensitize a solid-state imaging device, it is common to include a collecting element, described by such a name as a micro lens, in each pixel. Provision of the collecting element that focuses on a surface of a light-receiving element enhances efficiency of light collection into the light-receiving element. Meanwhile, light incident on pixels in the central region of a pixel area and light incident on pixels in a perimeter region of the pixel area differ in component. A great amount of light is incident onto the central region of the pixel area from the normal, whereas a great amount of light is incident onto the perimeter region of the pixel area from oblique direction. A typical collecting element is designed to collect the incident light from the normal into a focal point. For this reason, the collecting element can efficiently collect the incident light from the normal, while the collection efficiency for the incident light from oblique direction decreases. Thus, a problem arises that sensitivity of the pixel area decreases greater in the perimeter region than in the central region.
It is contemplated to dispose convex lenses having different shapes in the central region and perimeter region of the pixel area in order to ameliorate the problem that the sensitivity of the pixel area decreases toward the perimeter region. However, it is very difficult and impractical to achieve properly controlled positioning of the convex lenses in a matrix. For this reason, use of graded index lenses having periodic structures comprising different materials having different indices of refraction are discussed (e.g., see Japanese Unexamined Patent Application Publication No. 2006-351972).